The use of microporous multi-layered membranes as battery separators is known. See, for example, U.S. Pat. Nos. 5,480,745; 5,691,047; 5,667,911; 5,691,077; and 5,952,120.
U.S. Pat. No. 5,480,745 discloses forming a multi-layered film by co-extruding the multi-layered precursor or by heat-welding, at 152° C., pre-formed precursor layers. The multi-layered precursor, formed by either technique, is then made microporous by annealing and stretching. This membrane, which is made by a dry stretch process, has a preferable amount of net stretch is from 100% to 300%.
U.S. Pat. No. 5,691,047 discloses forming the multi-layered film by co-extruding the multi-layered precursor or by uniting, under heat (120-140° C.) and pressure (1-3 kg/cm2), three or more precursor layers. The precursor formed under heat and pressure, at a speed of 0.5 to 8 m/min (1.6-26.2 ft/min), has a peel strength in the range of 3 to 60 g/15 mm (0.2-4 g/mm). In the examples, one 34 μm separator has a peel strength of 1 g/mm and the other, about 0.5 g/mm. The multi-layered precursor, formed by either technique, is then made microporous by annealing and stretching. The porosity of these separators is greater than the present invention while showing a relatively high Gurley.
U.S. Pat. No. 5,667,911 discloses forming the multi-layered film by uniting (by heat and pressure or by adhesives) cross-plied microporous films to form a multi-layered microporous film. The microporous films are laminated together using heat (110° C.-140° C.) and pressure (300-450 psi) and at line speeds of 15-50 ft/min (4.6-15.2 m/min). This reference teaches lower Gurley values, which is a good indication that the porosity of these films is high.
U.S. Pat. No. 5,691,077 discloses forming the multi-layered film by uniting, by heat and pressure (calendering), or by adhesives, or by pattern welding, microporous films to form a multi-layered microporous film. Calendering is performed at 125° C. to 130° C. for a residence time of 2 to 10 minutes. Four (4) stacked multi-layered microporous precursors are calendering between a single nip roll. The porosity of these separators is greater than the present invention while showing a relatively high Gurley.
U.S. Pat. No. 5,952,120 discloses forming the multi-layered film by extruding nonporous precursors, bonding together nonporous precursors, annealing the bonded, nonporous precursors, and stretching the bonded, nonporous precursors to form a multi-layered microporous film. At least four (4) tri-layer precursors are simultaneously passed through the steps of bonding, annealing, and stretching. Bonding was performed between nip rollers at 128° C. (range 125° C.-135° C.) at a line speed of 30 ft/min (9.1 m/min) to yield a peel strength of 5.7 g (0.2 g/mm) and between nip rollers at 128° C.-130° C. at a line speed of 40 ft/min (12.2 m/min) to yield a peel strength of 30 g/in (1.2 g/mm). The net stretch on these separators all tend to be at least 100% or higher, while the Gurley's are on the high side.
While the foregoing processes have produced commercially viable multi-layered, microporous films suitable for use as battery separators, there is a desire on the part of both the separator manufacturers and the battery manufacturers to produce separators with greater processability. To improve processability a separator needs be more resistant to failure during the manufacture process. Two of the big problems that plague the battery manufactures are leaks and shrinkage of the separator. Shrinkage occurs when the separator is subjected to a heated environment, which a battery will go through during use. In the past one way separators had been tested for leaks was through a puncture strength test. However, it has been learned that a new test called mixed penetration, is by far a better indicator of how a separator will do in the manufacturing process than the puncture strength test. When testing for shrinkage the separator needs to be exposed to elevated heat over a time period. The manufactures of the batteries will still demand that the separators have Gurley numbers in a desirable range
Accordingly, there is a need to provide an improved multi-layered microporous films to be used as separators, which shows an increase in mixed penetration strength, while still maintaining low shrinkage values, and still exhibiting Gurley numbers in a desirable range.